Rechargeable battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Among all kinds of rechargeable batteries, lithium-ion battery (Li-ion battery or LIB) is the most commonly used one. It can be applied to many electric products, such as electric vehicles, electric motorcycles, electric tools, toys and so on. Stable power supply, compact size and recharging ability are the advantages for all applications.
Power management of lithium-ion batteries needs a very accurate voltage measuring circuit. No matter electric vehicles, electric motorcycles, electric tools, or toys, the application of lithium-ion batteries requires many lithium-ion batteries connected in series. Usually, an analog to digital converter for high voltage or several individual analog to digital converters are necessary components. However, each analog to digital converter has different characteristics from others. Namely, measurement of each analog to digital converter will be different from others. The analog to digital converters have to be calibrated before measuring processes begin. It is better if an analog to digital converter can be able to calibrate itself when it is working. In addition, cost for calibration of analog to digital converters is expensive. Power management of lithium-ion batteries still needs improvements.
In order to overcome the disadvantages, U.S. Pat. No. 7,554,291 disclosed a chargeable-and-dischargeable power supply system. Please refer to FIG. 1. The system includes a number of battery cell sections connected in series with one another; a number of cell state detecting sections installed for the respective battery cell sections and configured to detect a charge state in the respectively corresponding battery cell sections; a power control section configured to carry out a power supply control for the battery cell sections; a first electrical isolation section installed in a first signal route from the power control section to one of the cell state detecting sections which is for a highest potential positioned battery cell section; and a second electrical isolation section installed in a second signal route from one of the cell state detecting sections which is for a lowest potential positioned battery cell section to the power control section.
Next, please refer to FIG. 2. Another prior art is disclosed in US Patent Application No. 2006/0132139. An apparatus measures a voltage of a cell while scanning a group of cells in a cell stack, in which a number of cells is electrically connected in series. The apparatus has a first switching device and a voltage detecting device. The first switching device is connected in series with a signal line carrying a voltage of a cell. The voltage detecting device detects the voltage of the cell, which is electrically connected with signal lines carrying voltages of cells belonging to a group. When the first switching device is electrically connected with a connecting point between two successive groups of cells, the first switching device is shared by the two groups.
The aforementioned prior arts show some breakthroughs in cell voltage detection. However, they need external components and integrated circuits for voltage transformation. This brings additional cost. Meanwhile, self-calibration is not available. Although calibration problems can be solved by use of a calibrator connected to each battery cells, multiple calibrators will be needed for a battery pack which contains multiple battery cells which would undoubtedly increase the overall manufacturing cost. Furthermore, if a single calibrator is used for calibration of multiple battery cells, the overall voltage needed for the calibrator would be very high such that the manufacturing cost would increase magnificently. Hence, it is desperately needed for a cell voltage monitoring device which has the ability of self-calibration and which does not need to depend on multiple calibrators, and thereby reducing the overall manufacturing cost.